1. Field of the Invention
The present invention relates to thermal barriers, to a process and to a material for their production and to their applications.
2. Description of Related Art
Thermal barriers are essential constituents of many mechanical assemblies which withstand large temperature differences: turbocompressors, combustion engines, chemical reactors and the like. They are then intended to prevent excessive heating of certain, generally metallic, components whose surface temperature would, in their absence, exceed an acceptable limit, resulting in surface melting or else a deterioration in their properties.
Thermal barriers generally consist of an oxide or a mixture of oxides of very low thermal conductivity and are produced by deposition of the oxide or mixture of oxides by a chemical or physical route (for example cathode sputtering or plasma torch) on the substrate to be protected. The oxide most frequently employed is zirconia stabilized with yttrium oxide, which withstands very high temperatures. The deposition of zirconia is produced by plasma sputtering using a conventional technique starting with the powdered material. Zirconia exhibits a low thermal diffusivity (.alpha.=10.sup.-6 m.sup.2 /s). However, it has a relatively high density .rho., which constitutes a disadvantage in some applications: moreover, some of its mechanical properties, such as hardness and resistance to wear and to abrasion are low. Another oxide which can be employed as a thermal barrier is alumina stabilized with another oxide such as, for example, titanium dioxide TiO.sub.2. Alumina has a density which is lower than that of zirconia, and a diffusivity and a specific heat which are higher than that of zirconia, but its mechanical properties are not satisfactory.
The use of stainless steels and of certain refractory steels to form thermal barriers is also known. These offer thermal insulation properties but have a high density.
In the majority of devices equipped with thermal barriers the cyclic variation in temperature is the determining factor with regard to the lifetime and the reliability of the barrier. In fact, the latter is subjected to temperature variations of high amplitude (from the temperature at rest to the nominal operating temperature) and of long periodicity (for example several hours), but which can be produced over very brief periods as, for example, when a combustion engine is started up. A temperature modulation of smaller amplitude but very fast cycling is generally superimposed on these abrupt changes as, for example, in a motor vehicle engine, where the combustion/exhaust cycle takes place with a period of the order of a few tens of hertz.
This then results in a very great fatigue of the thermal barrier because of the cyclic mechanical stresses generated by the difference in thermal expansion of, on the one hand, the barrier support and, on the other hand, the material constituting the barrier. The damage very generally affects the thermal barrier rather than the underlying metal support, since the difference in expansion coefficient is likely to give rise to tensile stresses in the barrier, essentially when it consists of an oxide. The damage is concentrated in the barrier, near the substrate/barrier interface, where the shear stresses are highest. Cracks which are parallel to the interface are then produced and result in the separation of the barrier. The disappearance of its protective role can have a catastrophic effect, for example if it causes melting of the underlying metal. To limit or eliminate these disadvantages it has therefore been proposed to interpose a bonding layer consisting of a metallic material between the metal support and the thermal barrier. The choice of a metallic material exhibiting an expansion coefficient which is intermediate between that of the substrate and that of the material forming the thermal barrier makes it possible to reduce the temperature gradient at the interface. The choice of a metal alloy which becomes plastic in the temperature region in which the interfacial stresses can constitute a threat to the behaviour of the thermal barrier makes it possible in practice to eliminate the effects of the temperature gradient at the interface. The material most conventionally employed in the industry belongs to the class of the MCrAlY alloys where M denotes a metal such as nickel. However, the need to use a bonding layer in combination with the layer of material forming the actual thermal barrier presents a disadvantage insofar as the dimensions of the finished article subjected to thermal shocks cannot exceed certain upper limits. The addition of a bonding layer then results in a decrease in the dimensions of the substrate, and this can be detrimental to the other properties of the article.
A new category of alloys has more recently been proposed as a thermal protection component, which are quasicrystalline aluminum alloys whose thermal diffusivity is close to, or even lower than that of zirconia, and which become superplastic from 650.degree. C. onwards, some of them retaining this superplasticity up to about 1200.degree. C.
A quasicrystalline alloy is an alloy comprising one or more quasicrystalline phases which are either quasicrystalline phases within the strict meaning, or approximating phases (EP-A-521,138, Dubois et al). Quasicrystalline phases within the strict meaning are phases exhibiting symmetries of rotation which are normally incompatible with the symmetry of translation, that is to say symmetries of axis of rotation of order 5, 8, 10 and 12, these symmetries being disclosed by radiation diffraction. By way of example, the icosahedral phase of point group m35 and the decagonal phase of point group 10/mmm may be mentioned.
The approximating phases or approximating compounds are true crystals insofar as their crystallographic structure remains compatible with the symmetry of translation, but which on the electron diffraction pattern exhibit diffraction figures whose symmetry is close to the 5-, 8-, 10- or 12-fold axes of rotation.
Among these phases there may be mentioned by way of example the orthorhombic phase O.sub.1, characteristic of an alloy which has the atomic composition Al.sub.65 Cu.sub.20 Fe.sub.10 Cr.sub.5, whose unit lattice constants expressed in nm are: a.sub.0 (1)=2.366, b.sub.0 (1)=1.267, c.sub.0 (1)=3.252. This orthorhombic phase O.sub.1 is known as approximating to the decagonal phase. Furthermore it is so close to the latter that it is impossible to distinguish its x-ray diffraction pattern from that of the decagonal phase. It is also possible to mention the rhombohedral phase with constants a.sub.R =3.208 nm, .alpha.=36.degree., present in the alloys of composition close to Al.sub.64 Cu.sub.24 Fe.sub.12, based on the number of atoms. This phase is a phase approximating to the icosahedral phase. It is also possible to mention orthorhombic phases O.sub.2 and O.sub.3 with respective constants a.sub.0 (2)=3.83, b.sub.0 (2)=0.41, c.sub.0 (2)= 5.26 and a.sub.0 (3)=3.25, b.sub.0 (3)=0.41, c.sub.0 (3)=9.8, in nanometers, which are present in an alloy of composition Al.sub.63 Cu.sub.17.5 Co.sub.17.5 Si.sub.2 based on the number of atoms, and also the orthorhombic phase O.sub.4 with constants a.sub.0 (4)=1.46, b.sub.0 (4)=1.23, c.sub.0 (4)=1.24, in nanometres, which is formed in the alloy of composition Al.sub.63 Cu.sub.8 Fe.sub.12 Cr.sub.12, based on the number of atoms. It is also possible to mention a phase C, of cubic structure, very frequently seen to coexist with the approximating or true quasicrystalline phases. This phase, which is formed in some Al-Cu-Fe and Al-Cu-Fe-Cr alloys, consists of a superstructure, by chemical ordering of the alloy elements on the aluminum sites, the latter forming a Cs-C1 type structure and with lattice constant a.sub.1 =0.297 nm. A diffraction pattern for this cubic phase has been published for a sample of pure cubic phase of composition Al.sub.65 Cu.sub.20 Fe.sub.15, based on the number of atoms. It is also possible to mention a phase H of hexagonal structure, which is derived directly from the C phase, as shown by the epitaxy relationships observed by electron microscopy between crystals of the C and H phases and the simple relationships which link the crystal lattice constants, namely a.sub.H =3.sqroot.2.alpha..sub.1 /.sqroot.3 (to within 4.5%) and c.sub.H =3.sqroot.3.alpha..sub.1 /2 (to within 2.5%). This phase is isotypical of a hexagonal phase, written .PHI.AlMn, discovered in Al-Mn alloys containing 40% by weight of Mn.
The cubic phase, its superstructures and the phases derived therefrom constitute a class of phases approximating to the quasicrystalline phases of nearby compositions.
All these quasicrystalline alloys can be employed as a thermal barrier. In addition, by virtue of their superplasticity properties, in some cases up to about 1200.degree. C., they can furthermore be employed as a bonding layer for conventional thermal barriers consisting of oxides. However, the melting temperature of these alloys is lower than the temperatures reached by some articles to be protected during thermal cycles. The barrier is then destroyed. Consequently these materials are suitable for forming thermal barriers only for some uses.